1. Field of the Invention
The present invention relates generally a method and apparatus for dynamically adjusting the buoyancy of towed seismic arrays comprising a plurality of seismic sensors and specifically to the dynamic buoyancy control of a towed seismic array by means of activating electroactive polymers (EAP) incorporated within the towed seismic array. The bulk density of the seismic array is electrically altered to accomplish neutral, negative or positive buoyancy for a towed array of seismic sensors within the full range of water-densities, temperatures and salinities from fresh to extremely dense and saline seawater.
2. Summary of the Related Art
The related art discloses a wide variety of towed marine seismic tracking and positioning systems and methods typically comprising one or more seismic arrays (also known as streamers) and/or one or more seismic sources. Many of these seismic tracking and positioning systems and methods utilize a main or host vessel and/or other associated unmanned vessels or vehicles to tow the seismic array. Typically, towing is controlled or guided by a central control system. The known marine seismic tow tracking and positioning systems and methods are comprised of apparatuses such as seismic hydrophone streamers and attached floats, paravanes, and/or buoyant members. Typically each streamer and control apparatus is connected to the host vessel by a line, cable or tether. Considerable towing power is required of a host vessel to tow existing seismic streamer systems, cables and interconnecting sensing devices. A typical host vessel is capable of towing a plurality of seismic streamers. When the streamers are not deployed, they and their associated support apparatus are carried on the vessel""s deck. In some cases, multiple tow vessels will operate cooperatively to provide greater flexibility in the positioning and control of seismic streamers.
One such system for positioning and control of marine seismic streamers is taught in the international application published under the Patent Cooperation Treaty (PCT), International Publication Number WO 00/20895, international publication date Apr. 13, 2000. A marine seismic system with independently powered tow vehicles is taught by U.S. Pat. No. 6,028,817. A control device for controlling the position of a marine seismic streamer is taught in the international application published under the Patent Cooperation Treaty (PCT), International Publication Number WO 98/28636, international publication date Jul. 2, 1998.
With known seismic streamer towing, tracking and positioning systems, the location and spacing of system components is limited by the type, size, and length of cables used and by the characteristics of the towing vehicles and other control devices utilized by the known seismic tracking and positioning systems. The plurality of towed seismic streamers form an array which is towed behind the host vessel. Typically, changing the buoyancy and/or depth of such a towed streamer array, comprised of known components is a complex, cumbersome, time-consuming operation and can often become somewhat unwieldy.
A control device or xe2x80x9cbirdxe2x80x9d for controlling the depth and horizontal position of a seismic streamer is taught by WO patent application WO 98/28636, international publication date Jul. 2, 1998. The bird has two opposed wings which are independently controllable in order to control the streamers lateral position as well as its depth. These mechanical birds create turbulence which may interfere with the seismic signal sensed by the streamers during data acquisition. The mechanical birds also generate a tremendous amount of drag on the towing vessel which increases the amount of force needed to pull the streamer through the water during a data acquisition run. This increase in required force also increases the size and cost of the towing vessel. Additional horsepower and increased fuel capacity require larger and therefore more expensive tow vessels. Increased fuel consumption required to overcome the drag of mechanical birds increases the recurring costs of fuel.
The mechanical birds also must be manually attached to the seismic streamer cable and manually removed from the streamer during streamer retrieval. Attachment and removal of the mechanical birds is a labor-intensive, time-consuming process that adds significantly to the cost of data acquisition.
There are many problems associated with establishing and changing the buoyancy of streamers. During the design and fabrication of streamers, great care is taken to properly balance the amount of positively buoyant materials incorporated to offset the weight of negatively buoyant electrical and mechanical components to produce a bulk density very near to neutral buoyancy in seawater to a high degree of precision. Adjusting the buoyancy of a streamer to accommodate differences in seawater temperature and salinity requires addition or subtraction of ballasting weights. Seismic equipment is typically operated at relatively shallow water where the temperature and salinity are subject to significant variation with time and location. Seismic operations can span large distances and large areas. A streamer neutrally balanced for seawater at one time and place will not necessarily be balanced for other times and locations.
A streamer ballasted too heavily or too lightly requires the mechanical birds to provide a correcting force to position the streamer at the desired depth. The greater the imbalance, the greater the wing angle on the bird. Increased wing angles increase drag and acoustic noise. A streamer improperly ballasted may become uncontrollable if the birds cannot provide sufficient lift. Lift is proportional to the velocity of the streamer through the water. Streamer velocity may decrease due to a decreased velocity of the tow vessel. If the streamer should have no velocity (whether due separation from the tow vessel or a stopped tow vessel), the mechanical birds are incapable of producing forces to affect the depth of the streamer. Streamer velocity may also decrease during turns of the streamer array. For multi-streamer arrays, the streamers on the inside of the turn will move relatively slower than those on the outside of the turn.
If the streamer is ballasted too lightly, the streamer will float and the streamer velocity is too low to produce the force required to depress the streamer, it will float on the surface and require operator intervention to increase the ballasting of the streamer. Operator intervention takes significant time and effort that adversely affects the operational efficiency of the data acquisition system. If streamers are too heavily ballasted, they will sink if the mechanical birds cannot produce the lift required to control the depth of the streamer. Such loss of depth control may result in the irretrievable loss of the streamer resulting in a significant material loss and reduce operational efficiency of the seismic system.
Prior art solutions, designed to prevent streamers from diving too deeply, attached emergency floatation bags that filled with gas when onboard sensors detect an over-depth streamer condition. The gas to fill a buoyancy bag is provided by cylinders of compressed air or the byproduct of a pyrotechnic device. In addition to the increased cost of using buoyancy bags, they add to the effort and expense of deploying and retrieving streamers and add to the drag and acoustic noise provided by the streamer.
Finally, some streamers are fabricated with compressible materials to provide buoyancy. Compressible materials change buoyancy as a function of depth. This undesirable characteristic amplifies an imbalance in ballasting. A streamer ballasted too lightly becomes lighter the shallower it goes and a streamer ballasted too heavy becomes heavier as it sinks. These conditions add to the work required from mechanical birds and can result in the loss of depth control for the streamer.
Thus, there is a need for a method and apparatus that controls the depth of seismic streamers during data acquisition runs that creates less turbulence, less drag and enables dynamic control of the buoyancy of seismic streamers. There is also a need for a seismic streamer that floats when severed from the towing vessel.
The present invention provides a method and apparatus for dynamically adjusting the buoyancy of towed seismic arrays comprising a plurality of seismic sensors. The present invention enables dynamic buoyancy control of a towed array of seismic sensors by applying electric stimulus to electroactive polymers (EAP) incorporated within the streamer. The bulk density of the streamer is electrically altered to accomplish neutral buoyancy for a streamer within the full range of water-densities from fresh to extremely dense seawater. The present invention enables depth control of a towed array of seismic streamers under a variety of water density conditions that vary with time, temperature and location. The dynamic buoyancy control of the present invention enables towed seismic streamer depth control using fewer or no external mechanical depth control devices or birds attached to the streamer. The use of fewer external devices on the towed seismic streamer array creates less drag and less turbulence during towing, thereby providing a more efficient and quieter streamer array.